characteristics of heavy metals to various kind of biomass (Chen and Wang 2008).
Usually, biosorption is a metal ion physico-chemical interaction with
microorganisms’s particular cell surface groups that may increase or prohibit intracellular transfer or affect the transformation mechanism as well as biomineralization
which is extracellular precipitation (Davis et al. 2003; Gavrilescu 2004). Biosorption
is dependent on metabolism-independent passive or metabolism-dependent active
storage methods (Wang and Chen 2006), in mixtures that vary qualitatively as well
as quantitatively and are based on a variety of biomass, its source, practicability and
type of handing out (Veglio and Beolchini 1997). Metal accumulation processes can
be categorized on the basis of their dependence on cellular metabolism or according
to the position of the metal inside the cell.
12.5.4 Biosorption Mechanisms
The intricate structure of microorganisms indicates that microbial cells have several
routes for metal uptake. There are several biosorption processes which are still not
yet fully resolved. They can be categorized on the basis of several facts. Intracellular
storage of metals takes place through the metal transport across the cell membrane
depending upon cell metabolism meaning that this type of biosorption is possible
only in viable cells. Mostly, it is linked with an active microorganism’s defence
system, which responds to toxic metal presence. While in non-metabolism-based
biosorption, metal is taken up by physico-chemical reactions of metal with that of
functional groups of the microbial cell surface. This relies on physical adsorption,
ion transfer and chemical sorption and is independent of cell metabolism. Mainly,
polysaccharides, proteins and lipids are present in microbial cell walls with several
metal-binding groups like carboxyl, sulphate, phosphate and amino groups. This
non-metabolism-dependent biosorption is comparatively fast that can be reversed
back (Kuyucak and Volesky 1988). During precipitation, metals are taken up by the
solution as well as the cell surface (Ercole et al. 1994). In addition, it can be cell
metabolism-dependent if the microorganism generates compounds due to the existence of toxic metals favouring the precipitation process. Precipitation may not be
cell metabolism-dependent, if it is happening after a chemical reaction of the metal
with the cell surface. Because of biomaterial complexity, several of these
mechanisms may happen together in scales on the basis of biosorbent and environmental surroundings (Kefala et al. 1999; Volesky 2001; Gavrilescu 2004). Wang and
Chen (2006) have centred biosorption studies on the choice of best possible biomass
varieties whose prospects by sorption will be based on three areas, i.e. biology,
chemistry and engineering (Kefala et al. 1999). Biosorption potential is influenced
principally by three reasons: (1) properties of the metal ion like atomic weight, ionic
ray and valence; (2) environmental settings like pH, temperature, ionic strength,
contact period and biomass concentration; and (3) the biosorbent character which
possibly decides variations in choice as well as metal ions affinity because the
variety and microorganisms species, growth conditions, physiological state and
age influence the binding procedure of heavy metals (Wang and Chen 2006; Chen
12 Remediation of Heavy Metals Through Genetically Engineered Microorganism
325
Usually, biosorption is a metal ion physico-chemical interaction with
microorganisms’s particular cell surface groups that may increase or prohibit intracellular transfer or affect the transformation mechanism as well as biomineralization
which is extracellular precipitation (Davis et al. 2003; Gavrilescu 2004). Biosorption
is dependent on metabolism-independent passive or metabolism-dependent active
storage methods (Wang and Chen 2006), in mixtures that vary qualitatively as well
as quantitatively and are based on a variety of biomass, its source, practicability and
type of handing out (Veglio and Beolchini 1997). Metal accumulation processes can
be categorized on the basis of their dependence on cellular metabolism or according
to the position of the metal inside the cell.
12.5.4 Biosorption Mechanisms
The intricate structure of microorganisms indicates that microbial cells have several
routes for metal uptake. There are several biosorption processes which are still not
yet fully resolved. They can be categorized on the basis of several facts. Intracellular
storage of metals takes place through the metal transport across the cell membrane
depending upon cell metabolism meaning that this type of biosorption is possible
only in viable cells. Mostly, it is linked with an active microorganism’s defence
system, which responds to toxic metal presence. While in non-metabolism-based
biosorption, metal is taken up by physico-chemical reactions of metal with that of
functional groups of the microbial cell surface. This relies on physical adsorption,
ion transfer and chemical sorption and is independent of cell metabolism. Mainly,
polysaccharides, proteins and lipids are present in microbial cell walls with several
metal-binding groups like carboxyl, sulphate, phosphate and amino groups. This
non-metabolism-dependent biosorption is comparatively fast that can be reversed
back (Kuyucak and Volesky 1988). During precipitation, metals are taken up by the
solution as well as the cell surface (Ercole et al. 1994). In addition, it can be cell
metabolism-dependent if the microorganism generates compounds due to the existence of toxic metals favouring the precipitation process. Precipitation may not be
cell metabolism-dependent, if it is happening after a chemical reaction of the metal
with the cell surface. Because of biomaterial complexity, several of these
mechanisms may happen together in scales on the basis of biosorbent and environmental surroundings (Kefala et al. 1999; Volesky 2001; Gavrilescu 2004). Wang and
Chen (2006) have centred biosorption studies on the choice of best possible biomass
varieties whose prospects by sorption will be based on three areas, i.e. biology,
chemistry and engineering (Kefala et al. 1999). Biosorption potential is influenced
principally by three reasons: (1) properties of the metal ion like atomic weight, ionic
ray and valence; (2) environmental settings like pH, temperature, ionic strength,
contact period and biomass concentration; and (3) the biosorbent character which
possibly decides variations in choice as well as metal ions affinity because the
variety and microorganisms species, growth conditions, physiological state and
age influence the binding procedure of heavy metals (Wang and Chen 2006; Chen
12 Remediation of Heavy Metals Through Genetically Engineered Microorganism
325